In most MRI systems, a static magnet (for example, a superconducting solenoid) creates a substantially homogeneous nominally static magnetic field within a region to be imaged. The static magnetic field (and consequently the imaging area) may be within a cylindrical enclosure with the static magnet coiled around the cylinder or may be in an open access type environment where the static magnets are plate-like and on opposite sides of the imaging area.
The different types of MRI structures also use electromagnetic gradient coils near the static magnet structure. When proper electrical currents are passed through these gradient coils, the nominally static homogeneous field in the imaging area may be temporarily altered to produce a controlled gradient magnetic field in any of three orthogonal coordinate directions. The magnetic gradient coils generally take a shape similar to the static magnet pieces; that is, with plate-like static magnet pieces, the gradient coils are also plate-like and with cylindrical static magnet coils, the gradient coils are also cylindrical. Generally, in both the plate-like and cylindrical embodiments, three electromagnetic gradient coils (one for each of the three orthogonal directions X, Y, and Z) are provided as a series of conductive windings on or in a gradient coil former. During the MRI procedure, currents are rapidly applied and removed from these various gradient coils to create the requisite gradient magnetic fields. The rapid switching of current in the conductive windings of the gradient coils creates forces which manifest themselves as bending moments on the gradient coils and the formers around them. Each time the respective gradient coils are energized by applying the input current, the gradient coil former deforms, and each time the current is removed, the gradient coil former at least partially relaxes. This deformation and relaxation that occurs each cycle that a gradient coil is energized results in a continuous acoustical string of noise bursts within the imaging area.
During the MRI procedure, these energizing cycles occur rapidly and repetitively thus causing the gradient coil former to vibrate like the diaphragm of an acoustic speaker. Like in the speaker, the vibrations in the MRI gradient coil former generate acoustic noise, and in the case of the MRI system, this acoustic noise can be quite loud and unpleasant for the human within the imaging area.